Add regex docs to plugin.php page

plugin.php

@@ -103,7 +103,29 @@ function better_blacklist_display_form() {
             <input type="hidden" name="action" value="blacklist_domain" />
             <input type="hidden" name="nonce" value="$nonce" />
             <p>Enter domains to blacklist (one per line):</p>
-        <textarea class="blacklist-domains" cols="60" rows="15" name="blacklist_form">$blacklist_domains</textarea>
+
+            <details>
+                <summary>Advanced Usage (Regex)</summary>
+                <div>
+                    <p>
+                        You can use regular expressions (Regex) to define more complex patterns for blacklisting domains.<br> 
+                        To use Regex, enter your pattern between slashes (/).<br>
+                        
+                        For example, to block all subdomains of example.com, you could enter <code>/\.example\.com$/i</code>.<br>
+                        The <kbd>i</kbd> flag at the end makes the match case-insensitive. Usually you want to use this flag.
+                    </p>
+
+                    <p>
+                        <b>Further examples:</b><br>
+                        <code>/.*\.xxx$/i</code> - Blocks all domains ending with .xxx<br>
+                        <br>
+
+                        Be cautious when using Regex, as incorrect patterns can lead to unintended blocking of domains.<br>
+                        Always test your Regex patterns to ensure they work as expected.
+                    </p>
+                </div>
+            </details><br>
+            <textarea class="blacklist-domains" cols="60" rows="15" name="blacklist_form" placeholder="Example: block.example.com">$blacklist_domains</textarea>
             <p><input type="submit" value="Save" /></p>
         </form>
     HTML;